It is understood agents have also examined a call made to a senior aide of Crown Prince bin Salman by the team that carried out the killing.

Sources quoted in the US media stressed that there was no single piece of evidence linking the crown prince directly to the murder, but officials believe such an operation would have needed his approval.

What do the Saudis say happened to Khashoggi?

At a news conference in Riyadh on Thursday, Deputy Public Prosecutor Shalaan bin Rajih Shalaan said Khashoggi was given a lethal injection and his body was dismembered inside the consulate after his death.

The body parts were then handed over to a local "collaborator" outside the grounds, he added.

A composite sketch of the collaborator has been produced and investigations are continuing to locate the remains.

Eleven unidentified people have been charged over the journalist's death and the prosecutor is seeking the death penalty for five of them.

Damning case but no smoking gun

By Frank Gardner, Security Correspondent, BBC News

The reported CIA assessment that MBS ordered the killing of Jamal Khashoggi largely matches those in other Western capitals, including London.

The purported intercepted phone call - denied by the Saudis - between MBS's brother in Washington, Prince Khalid bin Salman, and Khashoggi, urging him to visit the Saudi consulate in Istanbul, would appear to be at the behest of the crown prince.

A second intercepted phone call was from the hit team on the day of the murder to someone inside MBS's inner circle in Riyadh. Again, it is hard to believe this would have been without the Crown Prince's knowledge

Taken together with the SIGINT (signals intelligence), the case against MBS is damning but still circumstantial.

Government officials do not believe there is a metaphorical "smoking gun" that explicitly ties MBS to the murder. But drawing on well-established diplomatic and intelligence contacts, they know that in that part of the world nothing gets done without sign-off from the top.

In the tightly controlled Arab Gulf states there is simply no such thing as "a rogue operation," as Saudi statements have described it. Meanwhile the glaring inconsistencies in the official Saudi explanations have only deepened suspicions of a state-sponsored cover-up.